Method to prevent slurry caking on cmp conditioner

ABSTRACT

A method of planarizing a semiconductor structure comprises moving a conditioning element on a surface of a polishing member, rotating the semiconductor structure relative to the polishing member against the surface of the polishing member, and rinsing the surface of the polishing member and the semiconductor structure. While the conditioning element is moved over the surface of the polishing member and the semiconductor structure is rotated against the surface of the polishing member, slurry is directed onto the polishing member. The step of rinsing comprises contacting the conditioning element to the surface of the polishing member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application 61/141,585, the entirety of which is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention generally relates to a method of planarizing a semiconductor structure. More particularly, the present invention relates to prevention of slurry caking on a chemical mechanical planarization (CMP) conditioner.

BACKGROUND

In chemical mechanical planarization or polishing (CMP) of semiconductor structures, in particular wafers, the wafer is pressed against a polishing pad by a polishing head. The polishing pad has an abrasive surface, which planarizes or flattens the surface of the wafer. This can be necessary before the wafer is subjected to further processes, such as photolithography. At the same time, a corrosive and abrasive slurry (containing sodium hydroxide, for example) is directed onto the wafer surface, which works in conjunction with the polishing pad to planarize the wafer. While the wafer is being polished or planarized, the polishing pad is itself conditioned by an abrasive conditioning disk that moves or rotates across the surface of the polishing pad. This removes material from the surface of the polishing pad so that it can planarize the wafer more effectively.

Slurry build-up can occur on the conditioning disk as slurry is allowed to dry on the disk. This slurry build-up has a two-fold negative impact on the CMP process. Firstly, as slurry fills the area between diamonds on the conditioning disk, the effectiveness of the disk to condition the polishing pad is reduced. Secondly, dried slurry particles build up on the disk and break loose during processing, resulting in scratches and particles on the wafer. Removing the dried slurry from the disk requires vigorous mechanical cleaning and/or the use of strong chemicals, which can damage the disk in such a way that it cannot be used again.

FIGS. 1 and 2 show two different types of CMP conditioning disks 12 after taking part in several CMP processes. As the conditioning disk 12 sweeps across the polishing pad during a CMP process, the conditioning disk 12 becomes covered with slurry on its abrasive surface that contacts the polishing pad. This is illustrated in FIGS. 1 and 2 as a dark area in the center of the conditioning disk 12. During processing, slurry that is present on the conditioning disk 12 does not have time to dry, but while the CMP apparatus is not operating, the conditioning disk 12 can dry and the slurry coated thereon can form a hard “cake”. Sodium hydroxide present in the slurry can also crystallize on the abrasive surface of the conditioning disk 12, which is very difficult to remove. It has been found that such an undesirable slurry build-up on the disk 12 occurs after processing as few as 100 wafers. However, it is generally required to use a single conditioning disk in CMP processing of more than 4000 wafers.

It has been proposed to use a “clean cup,” which keeps the conditioning disk wet between CMP processes so that the slurry does not have a chance to dry on the conditioning disk. However, when the conditioning disk is moved to the clean cup position at the end of a polish cycle, there is a significant time of about 2-8 seconds during which slurry can become dried on the disk. Therefore, use of a clean cup is inadequate for eliminating slurry build-up on the disk.

SUMMARY

The present invention provides a method of planarizing a semiconductor structure.

In one embodiment, the method comprises moving a conditioning element on a surface of a polishing member, rotating the semiconductor structure relative to the polishing member against the surface of the polishing member, and rinsing the surface of the polishing member and the semiconductor structure. Slurry is directed onto the polishing member at the same time as the conditioning element is moved on the surface of the polishing member and the semiconductor structure is rotated against the surface of the polishing member. During rinsing of the surface of the polishing member, the conditioning element is contacted to the surface of the polishing member. The conditioning element is used to condition the surface of the polishing member, or pad, that planarizes or polishes the surface of the semiconductor structure in contact with it. Conditioning of the polishing member takes place by moving a conditioning surface of the conditioning element across its polishing surface. The conditioning surface may be formed of particles of a hard, abrasive substance, for example diamond. At the same time, slurry is directed onto the surface of the polishing member, which also helps in the planarizing process of the semiconductor structure. The slurry may contain a caustic substance such as sodium hydroxide, for example. After the surface of the polishing member is conditioned, it is rinsed to remove slurry and particles removed from it during the conditioning process. While the polishing member is being rinsed, the conditioning element is also held in contact with the surface of the polishing member so that it, too, is rinsed. This means that the slurry is washed away from the surfaces of the conditioning element before it is exposed to air and allowed to dry. Advantageously, the temperature of the conditioning element is reduced by the rinsing step while it is held in contact with the polishing pad during rinsing of the polishing pad and the semiconductor structure. Cooling the conditioning element during the rinsing step also helps to reduce the rate at which slurry may dry, and therefore build up, on its surface.

The slurry build-up that occurs on prior art conditioning elements is prevented with the abrasive element in the method of the present invention. Thus, no particles break loose from the conditioning element and the semiconductor structure is not caused to be scratched or damaged. There is also no need to use strong chemicals or vigorous mechanical cleaning to remove dried slurry from the conditioning element and therefore the conditioning element is reusable in further CMP processes. This means that the method according to the present invention is highly advantageous with respect to prior art CMP processes.

Preferably, moving the conditioning element on the surface of the polishing member comprises rotating the conditioning element relative to the polishing member. The conditioning element can be mounted so that it is rotatable relative to the surface of the polishing member. This means that during conditioning of the polishing member, the conditioning element can be rotated on the surface of the polishing member, or the conditioning element can remain stationary while the polishing member rotates, or both the conditioning element and the polishing member may rotate simultaneously.

The step of rotating may comprise rotating both the semiconductor structure and the polishing member. The semiconductor structure and the polishing member may be rotated in the same direction, for example anticlockwise.

The step of contacting may further comprises oscillating the conditioning element against the surface of the polishing member. For example, the conditioning element may be configured so that its surface is oscillated backwards and forwards across the surface of the polishing member while the polishing member is rotating. Alternatively, or in addition, the step of contacting may further comprise rotating the conditioning element relative to the polishing member. The conditioning surface of the conditioning element may then be rotated against the surface of the polishing member.

The method according to the present invention is particularly advantageous when the semiconductor structure is a wafer. This is because wafers are highly susceptible to damage during CMP processing, when slurry that is caked on the conditioning element can break loose and scratch the wafer surface. In the method according to the invention, dried slurry does not build up on the conditioning element in the first place and therefore there are no particles that can break loose from the conditioning element and scratch the wafer.

The present invention also provides apparatus for planarizing a semiconductor structure. The apparatus comprises a polishing member having a surface for polishing the semiconductor structure, a conditioning element movable across the surface of the polishing member for conditioning the surface of the polishing member, means for directing slurry onto the surface of the polishing member, and rinsing means for rinsing the surface of polishing member and the semiconductor structure. Further, the apparatus comprises means operable to hold the conditioning element in contact with the surface of the polishing member when the rinsing means is rinsing the surface of the polishing member and the semiconductor structure. The conditioning element then remains in contact with the polishing member while the polishing member is rinsed. The high volume of water delivered during the rinsing process removes the slurry from the conditioning element before it has a chance to solidify. Furthermore, rinsing helps to reduce the temperature of the conditioning element, which means that the slurry does not have a chance to solidify on the conditioning element before it is removed from the surface of the polishing member. In this way, there are no slurry particles that can break loose from the conditioning element during polishing and damage the semiconductor structure. In addition, the conditioning element does not have to be subject to cleaning with harsh chemicals and is thus reusable. Therefore, the planarizing apparatus of the present invention is highly advantageous.

Preferably, the conditioning element is disk-shaped to form a conditioning disk. The polishing member and the conditioning element may be rotatable relative to each other. A surface of the conditioning element adapted to contact the polishing member may be provided with abrasive particles. For example, particles of a hard substance such as diamond may be impregnated into the surface of the conditioning element.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features of the invention will become apparent from the below description of a preferred embodiment, and from the accompanying drawings, in which:

FIG. 1 (Prior Art) shows a first type of abrasive conditioning disk for conditioning a polishing surface of a CMP polishing element after taking part in prior art CMP processing;

FIG. 2 (Prior Art) shows a second type of abrasive conditioning disk for conditioning a polishing surface of a CMP polishing element after taking part in prior art CMP processing; and

FIG. 3 is a simplified schematic of a CMP apparatus for carrying out the method according to the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 3 shows a CMP planarizing apparatus for carrying out the method according to the invention. A polishing pad 10 is generally disk-shaped and has an upper surface that is adapted to polish a semiconductor wafer 11. A conditioning disk 12 is provided for conditioning the polishing pad 10 and is affixed to a support arm 13, which can move so as to oscillate the disk 12 from the edge to the center of the polishing pad 10 during operation. A slurry arm 14 is adapted to direct slurry onto the upper surface of the polishing pad 10. The slurry contains a corrosive solution, for example sodium hydroxide.

During operation, the wafer 11 is pressed against the upper surface of the polishing pad 10 by a polishing head (not shown here), so that the surface of the wafer 11 in contact with the polishing pad 10 is planarized or flattened in the desired manner. The polishing head is rotatable, so as to rotate the wafer 11 against the planarizing surface of the polishing pad 10. The polishing pad 10 also rotates in the same direction as the wafer 11, for example anti-clockwise. At the same time, slurry is directed onto the polishing pad 10 by the slurry arm 14. The polishing pad 10 may be adapted to rotate eccentrically, depending on the requirements for planarization of the wafer 11.

During processing of the wafer 11, the conditioning disk 12 rotates to condition the polishing pad 10 and the support arm 13 moves the disk 12 from the edge to the center of the surface of the polishing pad 10 in an oscillating manner so that the entire surface of the polishing pad 10 is conditioned. The surface of the conditioning disk 12 that is in contact with the upper surface of the polishing pad 10 is impregnated with particles of a hard substance, for example diamond, to form an abrasive surface. As the disk 12 rotates across the surface of the polishing pad 10 and oscillates from the center to the edge of the polishing pad 10, this abrasive surface acts to condition the polishing pad 10, so that it can more efficiently planarize the wafer 11. The disk 12 can also be made to rotate in the same direction as the polishing pad 10 and the wafer 11. In this case, the path of slurry flow from the slurry arm 14 will be in the direction of rotation of the polishing pad 10, the wafer 11 and the conditioning disk 12. This is indicated by the large arrow in FIG. 3. In this example, the direction of rotation of the polishing pad 10, the wafer 11 and the disk 12, as well as the direction of the slurry flow, is anti-clockwise.

TABLE 1 Step 1 Step 2 Step 3 Step 4 Polishing pad On On On On rotation Wafer rotation On On On On Wafer pressure Off High Low Low Conditioning On On On Off disk contact to polishing pad Slurry On On Off Off Water Off Off On On

The relative states of the components of the apparatus shown in FIG. 3 while it is performing the method of the present invention are shown in Table 1. Step 1 is the start-up of the apparatus, where the polishing pad 10, the wafer 11, and the disk 12 are all set to rotate. Step 2 is the processing step where planarization of the wafer 11 takes place. Rotation of the conditioning pad 10, the wafer 11 and the disk 12 remains switched on and the disk oscillates from the edge to the center of the conditioning pad 10. The wafer 11 is pressed against the surface of the polishing pad 10 and the abrasive action of the polishing pad 10, in conjunction with the corrosive slurry directed onto the wafer 11 from the slurry arm 14, planarize the surface of the wafer 11 that is in contact with the polishing pad 10. At the same time, the conditioning disk 12 acts to condition the polishing pad 10. In step 3 of the method, the slurry is switched off and water is directed onto the polishing pad 10 so as to rinse the polishing pad 10, the wafer 11 and the disk 12. During the time when water is being directed onto the polishing pad 10 in this rinsing step, the support arm 13 supporting the disk 12 is adapted to hold the disk 12 in contact with the polishing pad 10. A high volume of water is delivered to the polishing pad 10 during the rinse step and this removes the slurry from the disk 12 before the slurry solidifies. Furthermore, the water cools the disk 12. During the rinsing step, the pressure of the wafer 11 against the surface of the polishing pad 10 is reduced. In the fourth step of the process, the disk 12 is removed from the surface of the polishing pad 10 by the support arm 13 so that it is no longer in contact with the polishing pad 10. Since the water delivered to the disk 12 during the rinsing step cools the disk 12, in the event that there is any slurry remaining on the disk 12 after step 3 of the process, it will not have an opportunity to dry and build up on the disk 12 during step 4.

Although the invention has been described hereinabove with reference to a specific embodiment, it is not limited to this embodiment and no doubt further alternatives will occur to the skilled person that lie within the scope of the invention as claimed. 

1. A method of planarizing a semiconductor structure, the method comprising: moving a conditioning element on a surface of a polishing pad relative to the surface; rotating the semiconductor structure relative to the polishing pad against the surface of the polishing pad; directing slurry onto the polishing pad during the steps of moving and rotating; and rinsing the surface of the polishing pad and the semiconductor structure, wherein the step of rinsing comprises contacting the conditioning element to the surface of the polishing pad.
 2. The method according to claim 1, wherein the step of moving comprises rotating the conditioning element relative to the polishing pad.
 3. The method according to claim 1, wherein the step of rotating comprises rotating both the semiconductor structure and the polishing pad.
 4. The method according to claim 1, wherein the step of contacting further comprises oscillating the conditioning element against the surface of the polishing pad.
 5. The method according to claim 1, wherein the step of contacting further comprises rotating the conditioning element relative to the polishing pad.
 6. The method according to claim 1, wherein the semiconductor structure is a wafer.
 7. The method according to claim 1, wherein the semiconductor structure is a wafer; the step of moving comprises rotating the conditioning element relative to the polishing pad; the step of rotating comprises rotating both the semiconductor structure and the polishing pad; and the step of contacting further comprises oscillating the conditioning element against the surface of the polishing pad and rotating the conditioning element relative to the polishing pad.
 8. A method of planarizing a semiconductor wafer using chemical mechanical polishing (CMP), comprising: pressing a surface of the wafer against an abrasive surface of a rotating polishing pad and rotating the wafer relative to the polishing pad to planarize the wafer surface; conditioning the polishing pad by rotating a conditioning surface of a conditioning disk against and relative to the polishing pad abrasive surface to remove material from the polishing pad abrasive surface; the conditioning disk being supported on a support arm that moves the conditioning disk from an edge to a center of the polishing pad as the polishing pad rotates, to move the disk in an oscillating manner relative to the polishing pad abrasive surface; directing a slurry containing a corrosive solution from a slurry arm onto the abrasive surface of the rotating polishing pad, the slurry flowing from the slurry arm in the direction of rotation of the polishing pad to the conditioning surface and wafer surface; turning the slurry off and directing water onto the abrasive surface of the polishing pad for rinsing to remove slurry and polishing particles from and to cool the polishing pad abrasive surface, conditioning surface and wafer surface; while the polishing pad abrasive surface is being rinsed, reducing the pressure of pressing the surface of the wafer against the polishing pad abrasive surface, and holding the conditioning surface in contact with the polishing pad abrasive surface to prevent drying of the conditioning surface; and after rinsing and cooling the conditioning surface, removing the conditioning disk away from the polishing pad with the support arm.
 9. Apparatus for planarizing a semiconductor structure, the apparatus comprising: a polishing member having a surface for polishing the semiconductor structure; a conditioning element; a support arm supporting the conditioning element for movement across the surface of the polishing member for conditioning the surface of the polishing member; and a slurry arm for directing slurry onto the surface of the polishing member during polishing and for directing water onto the surface of the polishing member for rinsing; the support arm serving to hold the conditioning element in contact with the surface of the polishing member when thewater is rinsing the surface of the polishing member.
 10. The apparatus according to claim 9, wherein the polishing member and the semiconductor structure are rotatable relative to each other.
 11. The apparatus according to claim 9, wherein the conditioning element is disk-shaped.
 12. The apparatus according to claim 9, wherein a surface of the conditioning element in contact with the surface of the polishing member comprises abrasive particles.
 13. The apparatus according to claim 9, wherein the polishing member and the semiconductor structure are rotatable relative to each other; the conditioning element is disk-shaped; and a surface of the conditioning element in contact with the surface of the polishing member comprises abrasive particles. 